Mouseport element should have "border: 0" to work properly in IE6-8. By default borderLeftWidth is "medium" in IE, resulting in "NaN" css property calculation. Documentation does not mention this.
Good catch. I'd prefer not to force the designer to have 0 border on the mouseport. I'll investigate a solution that takes account of border width (using outerHeight / outerWidth as measurements would be a good start).